Solid state light (SSL) emitting devices, including solid state lamps having light emitting diodes (LEDs) are extremely useful, because they potentially offer lower fabrication costs and long term durability benefits over conventional incandescent and fluorescent lamps. Due to their long operation (burn) time and low power consumption, solid state light emitting devices frequently provide a functional cost benefit, even when their initial cost is greater than that of conventional lamps. Because large scale semiconductor manufacturing techniques may be used, many solid state lamps may be produced at extremely low cost.
In addition to applications such as indicator lights on home and consumer appliances, audio visual equipment, telecommunication devices and automotive instrument markings, LEDs have found considerable application in indoor and outdoor informational displays.
With the development of efficient LEDs that emit blue or ultraviolet (UV) light, it has become feasible to produce LEDs that generate white light through phosphor conversion of a portion of the primary emission of the LED to longer wavelengths. Conversion of primary emissions of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. This system for producing white light by combining an unconverted portion of the primary emission with the light of longer wavelength is well known in the art. Other options to create white light with LEDs include mixing two or more colored LEDs in different proportions. For example, it is well known in the art that mixing red, green and blue (RGB) LEDs produces white light. Similarly, mixing RBG and amber (RGBA) LEDs, or RGB and white (RGBW) LEDs, are known to produce white light.
The use of reflective surfaces is also well known in the art. Reflective surfaces have been used to direct light from the LED to the down-conversion material and/or to reflect down converted light which is generated from the down-conversion material. Even with these improvements, the current state of the art LED technology is inefficient in the visible spectra. The light output for a single LED is below that of known incandescent lamps, which are approximately 10 percent efficient in the visible spectra. To achieve comparable light output power density to current incandescent lamps, an LED device often requires a larger LED or a design having multiple LEDs. However, designs incorporating a larger LED or multiple LEDs have been found to present their own challenges.
Recent studies have determined that the heat generated from LEDs decreases overall light emission and bulb durability. More particularly, the LED device becomes less efficient when heated to a temperature greater than 100° C., resulting in a declining return in the visible spectra. Extended exposure to high heat also reduces the effective life of the LEDs. Additionally, the intrinsic down conversion efficiency for some down conversion phosphors also drops dramatically as the temperature increases above approximately 90° C. threshold.
Attempts to overcome these deficiencies have been focused on bulb designs that are unlike traditional incandescent lamps. The use of heat sinks at the base of the bulb has assisted in heat dissipation, but has led to lamp designs having significantly different aesthetics and light distribution functionality from traditional incandescent lamps. Even though solid state light emitting devices have been advancing rapidly and have exceeded the luminous efficacy of traditional A-lamp incandescent bulbs, there are no SSL-based replacement light bulbs that can produce light levels similar to incandescent lamps, have very high luminous efficacy values, and much longer life time. Thus, there is a particular need for solid state light emitting devices which can replace traditional incandescent lamps by providing similar or improved performance efficiency, life span durability, and bulb aesthetics.